Particulate Formation in Gasoline Direct Injection Engines

Licentiate thesis, 2019

Gasoline direct injection (GDI) engines are facing a great challenge because of the need to comply with increasingly stringent emission regulations while improving fuel economy. GDI engines are popularly known for their high fuel efficiency (by the standards of gasoline engines) and low emissions, enabling higher compression ratios and thus increased volumetric efficiency. Unfortunately, GDI engines tend to produce higher particulate number (PN) emissions than conventional port fuel injection (PFI) engines, mainly due to the challenges of in-cylinder liquid fuel injection. Cold starts, transients, and high loads account for a disproportionately high share of all PN emissions from GDI engines over a certification cycle. Understanding the mechanisms of PN formation during these stages is necessary for the further market penetration of GDI under the constraint of tighter emission standards. This knowledge becomes especially important when in future particles with sizes smaller than 10 nm are measured and legislated.

This work presents experimental investigation of particulate emissions from a naturally aspirated single cylinder metal gasoline engine operated in a homogeneous configuration. The engine was modified to be capable of operating using DI, PFI, or both simultaneously. PFI was configured with a custom inlet manifold to inject about 50 cm upstream of cylinder head, forming a more homogeneous fuel-air mixture than would otherwise be possible. The experimental campaigns were structured to systematically isolate and study different PN formation mechanisms. Mixing quality was improved  ubstantially by using a small amount of upstream injection together with direct injection and could be controlled by varying the mass split between the direct and upstream injectors. It was found that using a small upstream injection when operating in GDI mode could reduce PN emissions by up to a factor of 10 while only modestly increasing fuel consumption.

The chemical composition of the fuel could also strongly affect particulate emissions. Therefore, to find alternative ways of reducing PN emissions, experiments were conducted using a gasoline engine with fuel blends containing renewable oxygenates – either 10% (v/v) ethanol (EtOH) or 22% (v/v) ethyl tert-butyl ether (ETBE). It was observed that PN emissions was reduced using oxygenated fuels at low load for both PFI and DI operation, but not at higher loads where PN increased instead. Measurements of solid PN (SPN) emissions revealed that more soot was formed at high load along with an increase in emissions of volatile organic compounds (VOC).

PN measurements were conducted using a DMS500 fast particle spectrometer supplied by Cambustion. In addition, solid particulate measurements were performed by passing exhaust samples through a thermodenuder and a catalyst to remove most of the volatile organic compounds (VOCs) from the raw emissions. The results indicated that wall-wetting is the dominant particulate formation mechanism inside the cylinder: fuel-wall interactions with the piston, cylinder walls, and valves during the fuel injection period account for a significant fraction of the PN content of raw exhaust.